It is often desirable to inspect generally flat uniform surfaces for defects. Surface inspection is used in a variety of industries. The inspection process can utilize automated machine vision techniques, manual inspection techniques, in which a human operator views objects through a remote camera, or a combination of manual and automated techniques.
By way of example, the production of laminated security cards, sometimes referred to as "Smart Cards," involves the placement of a radio frequency-generating material between two layers of plastic sheet laminate. The finished cards are used in a variety of readers in which the response of the card to a predetermined radio frequency signal, transmitted by the reader, uniquely identifies the cardholder. Because of the substantial heat and pressure used to produce such cards, they are susceptible to the formation of defects on their surfaces during manufacture. These defects, which cause problems during the printing process, may be difficult to detect during visual inspection by a human operator, or even by automated inspection. In particular, the so-called dimple defect, which exhibits a gradual waviness emanating from a point on the card surface may be quite difficult to detect, and will only show up when the card is printed upon. Other types of defects may be equally difficult to spot, particularly when viewed using a remote camera that displays the card on a monitor.
Visualization of these defects can be made simpler by the use of structured illumination, where a pattern of light and shadow projected upon the surface enhances the visibility of any defects present. An effective and reliable way to determine whether surface defects are unacceptable is to measure the structured illumination grid features against predetermined tolerance values. This generally entails the numerical quantification of the underlying defects. However, accurate and reliable translation of the visual quality of a defect into a quantitative value is often difficult to accomplish. It is, therefore, an object of this invention to provide a system and method and apparatus for quantitatively inspecting a surface for defects. This system and method should enable easier and more reliable inspection by automated machine vision systems.